The invention concerns a method for the production of printed circuit boards including the following steps;
Production of recesses and/or through holes in a substrate made from insulating material, in particular by means of laser-ablation, wherein the recesses and/or through holes correspond to the desired structure of conductive tracks and/or through hole platings.
introduction of a base layer on one or both sides of the substrate; and PA0 introduction of a conductive material onto the base layer, the base layer being catalytic and/or activating with respect to the conducting material.
A manufacturing procedure of this type is e.g. known in the art through EP 0 677 985 A1.
When producing printed circuits or printed circuit boards, one distinguishes in principal between the widely used subtractive method which starts with metal-clad substrates or base materials and with which copper not needed for conducting leads is removed by means of etching, and the additive method which builds up the conducting material on bonding substrate layers in such a manner that following removal from the baths, the conducting material is only located where conducting leads are needed.
Combinations of these methods are also wide spread. In this manner the through hole contacting, i.e. by copper coating of the walls of the holes, additively processes conducting leads located on both sides and produced in a subtractive fashion. A semi-additive method uses galvanic amplification to build up the conducting leads on thin base layers produced by currentless deposition and the remaining base layers are removed through etching, i.e. subtractively. In order to produce through holes and bottom holes, mechanical boring is normally utilized in combination with direct metallization.
Known in the art through the German magazine publication "Galvanotechnik" [Galvanometric Technology] 77, (1986), No. 1, pages 51 through 60, is the use of a laser to assist the transfer of the conductive pattern during production of printed circuit boards using the fully additive method.
In addition EP A 0 164 564 discloses the possibility of producing bottom holes in a substrate using an excimer laser. This process of material removal is designated laser-ablation.
EP 0 677 985 A1 describes the production of recesses and through holes in substrates by means of ablation with the assistance of an excimer laser. Energy calculations allow for the production of a defined depth profile. A layer of electrically conducting material is subsequently introduced onto the surface of the substrate, e.g. using a PVD procedure. An additional metal layer is deposited on this base layer and the metal layer and the base layer are removed by grinding or polishing from the outer side of the substrate, in such a fashion that conductive material only remains in the recesses and/or through holes.
In this conventional method, the base layer is completely coated with a metal layer, e.g. a copper layer which must then be nearly completely removed by grinding, polishing, lapping or the like. This mechanical removal of the superfluous material causes high mechanical loading of the foil-like substrate. In addition a flat mechanical removal in a tolerance range on the order of magnitude of 1 .mu.m, requires very expensive installations.
EP 0 287 843 B1 discloses preferential use of a pulsed excimer laser for ablation of the base layer. Also disclosed in EP 0 287 843 B1 is the possibility of producing a negative image of the conductive leads with ablation by means of electromagnetic radiation, wherein the base layer is selectively removed without encroaching substantially on the surface of the substrate. Metallization cannot occur during subsequent processing steps in the regions at which the electromagnetic radiation has acted. In this manner, the advantages of the additive method are maintained in a particularly simple method of conductive pattern transfer using electromagnetic radiation.
This method known in the art from EP 0 287 843 B1 has the disadvantage that formation of the conductive tracks takes place on the surface of the substrate. This means that the combination of a plurality of layers results in a surface topography which causes problems when inserting the components into the printed circuit board.
It is therefore the purpose of the present invention to create a method for the production of printed circuit boards using the additive method which produces high quality technical results in an economical manner without harming the environment.
This purpose is achieved in accordance with one aspect of the invention in that that the base layer is selectively removed from the substrate except in the recesses and/or in the through holes prior to introduction of the conductive material.